Edfluorination of phosphate rock



offluorine to the desired relative amounts of CaO,

mol ratio of these con 2,839,377 Patented June 17, 1958 2,839,377 DEFLUORINATION on PHOSPHATE ROCK Clinton A. Hollingsworth, Lakeland, Fla., and John C. Williams, Catasauqua, Pa, assignors, by mesne assignments, to Smith-Douglass Company, Incorporated, Norfolk, Va., a corporation of Virginia No Drawing. Application February 28, 1951 Serial No. 213,284

4 Claims. (CI. 71-45) that the temperature required for the removal of fluorine is so near the fusion temperature of the rock that objecor sintering takes place before the removal extent is elTected. Hence, it is not practical to operate at the optimum defluorinating temperature unless some means is provided to prevent fusion at that and somewhat higher temperatures, because at times the operating temperature (particularly in a rotary kiln) will inadvertently rise above the fusion temperature. It has long been recognized that silica and water vapor aid defluorination, but silica in excess of about 4% and up to about lowersthe fusion temperature of the-calcining charge. It has further been recognized that alkali compounds (e. g. sodium carbonate) and phosphoric acid exert some beneficial eliect upon defiuorination, but these reagents too, like silica, tend to lower the fusion temperature when present in amounts contributing beneficially to fluorine removal.

in an extensive investigation of the effects of silica, phosphoric acid and alkali compounds in promoting the removal of fluorine from phosphate rock and in influencing the fusion temperature of the calcining charge, we have determined that by properly proportioning the relative amounts of the three reagents included in the calcining charge, and with due consideration of the grade (P 0 content) and lime (CaO) content of the phosphate material, substantially complete defiuorination can be effected by calcination in kilns of commercial size without objectionable fusion or sintering of the calcining charge. The present invention accordingly involves defluorinating phosphate rock by calcination at a temperature of at least 2600 F. without substantial fusion in the presence of water vapor and of a reagent mixture consisting essentially of the reaction product of phosphoric acid and sodium carbonate, with a mol ratio of Na O to P 0 in the reaction product between 1.6 and 2.8, and with such N320, P205 and combined phosphate rock and reagent mixture that the ituents is between 1.3 and 2.8 in

the formula ,MOIS CaO+Na O-3P O Mols SiO More particularly, the method of the invention involves reacting sodium carbonate with an aqueous solution of phosphoric acid in .Na O to P 0 in the reaction product is between 1.6 and 2.8., and such amount of the reaction product is mixed with the phosphate rock (containing from 2 to 6% of silica) that the calcining charge is made up of from 5 to "15% by weight of added Na O and P 0 in the form such proportions that the mol ratio of v;

rock containing about 35% from 4to 5% with-Phosphoric acid (as well as *of the reaction product and the balance (-85% weight on a dry basis) of the phosphate rock.

In practicing the invention in a rotary kiln, the phosphaterock is introduced into the feed orcold end of the kiln along with the reagent mixture in the form of the reaction product resulting from reacting sodium carbonate with an aqueous solution Commercial light soda ash having a Na O content of about 58% (over 98% Na CO is a satisfactory formof sodium carbonate. Crude commercial phosphoric acid resulting from reacting sulphuric acid with a phosphatebearing material, e. g. phosphate rock, is a satisfactory form of phosphoric acid. Conveniently, a predetermined amount of commerical soda ash is dissolved in a predetermin'ed amount of water in a suitable reaction tank, and a predetermined amount of an aqueous solution of crude phosphoric acid is then run into the reaction tank with constant stirring and in such gradual quantities as to avoid cog'ectionable foaming. The reaction product may advantageously be run into the feed or cold end of the kiln and mixed there with the rock by the tumbling action resulting from the rotation of the kiln. The phosphate rock customarily contains 10 to 14% of moisture, and such additional amount of water may advantageously be introduced into the kiln, along with the rock and reagent mixture, as to form at the cold end of the kiln a slurry containing from 30 to 40% of water.

"In the present preferred practice of the invention, the calcining charge is made up approximately 94-93% by weight of phosphate rock (on a dry basis) and approximately 6 7% by weight of added Na O and P20 in the form of the reaction product in which the mol ratio of Na O to P 0 is approximately 2.3. With a phosphate P 0 about 50 CaO and of silica (generally determined as insoluble matter in the phosphate industry), the mol ratio of the constituents of the calcining charge represented by the formula: t

CaO-|- Na O3P O Mols SiO will then be approximately 1.6 to 2.0. In practice, it is requently more convenient to determine and express ferred calcining charge, as fed to the kilmis made up as follows:

Percent. by weight Phosphate rock (on a dry basis) 93-10 Na O(added) 3.4 P 0 (addcd) 3:6

The boarder mol ratio of Na i) to P 0 of from 1.6 to .218 corresponds to a weight ratio of Na i) to P 0 of from 0.7 to 1,2.

The aqueous-solution of phosphoricacid may be of any desiredconcentration. Advantageously, the concentration of the chemical compounds in the reaction product may be around20%. Since sodium compounds other than the carbonate (e. ,g. the bicarbonate hydroxide .etc.) react other acid phosphate compounds) to produce reaction products having an Na Q/ P O mol ratio of from 1.6 to 2.8, such other sodium compounds and acid phosphate compounds are within the principle of the invention, and sodium carbonate and fphosphoric acid in the appended clairns=are intendedto include such obvious equivalents. Similarly, potassium compounds may be substituted in whole or in part for equivalent sodium compounds without departing from the principles of the invention.

mesh and plus 28 mesh) and with fiotation concentrate'(most ly minus vrial may be included in trates, mostly minus 14 mesh and plus 150 mesh).

tion is effected.

hour gives a satisfactory i reagent mixture The phosphate rock may be any of the usual commercial products of a phosphate rock mill or concentrator. Excellent results have been obtained in the practice of the invention on a commercial scale with washer plant screen product (mostly-i. e. at least 90%--minus 6 belt or table-type mesh and plus 65 mesh). The particle size of the rock is not especially critical, and a larger proportion of fine particle size matethe calcining charge, as contrasted with other defluorinating processes in which rock of fine particle sizeis troublesome. Thus, in practicing the invention, it is possible to include in the calcining charge substantial amounts of froth flotation concentrate (mostly minus mesh and plus 150 mesh), and plant concentrate (combined belt and froth flotation concen- Generally speaking, it is desirable in defluorinating processes. in which phosphoric acid is included in the calcining charge to use as much fine particle size rock as possible, and the possibility of doing so is an advantage of the invention. It has been found that a coarse feed (e. g. washer plant screen product) is usually discharged from i a rotary kiln with a finer particle size than the feed, while a. fine feed -(e. g. froth flotation concentrate) is usually discharged with a coarser particle size than the feed.

Calcination is most conveniently carried out in a rotary kiln, although other types of calcining equipment may 7 'be used. Calcination is conducted in the presence of water vapor, care being exercised to assure intimate and continuous association of water vapor with the entire body of the charge until substantially complete defluorina- The calcining temperature should ultimately be sufficiently high to eliminate substantially all .of the fluorine and to impart high fertilizer availability to the phosphate content of the calcined product, and to this end should be at least 2600 F. and may be as high as 2800 F. Calcination is carried out in the absence of substantial fusion or sintering of the charge. A detention period of 10 to 20 minutes at approximately the ultimate calcining temperature, e. g. the hot zone of a rotary kiln, is sufficient to substantially defluorinate the rock and impart high fertilizer availability to its phosphate content.

In a rotary kiln, the depth of charge should be such as to insure adequate penetration of water vapor and escape of evolved fluorine. Too deep a bed of charge impairs these requirements and results in poor defluorina-.

carried out in a single pass through a rotary kiln 6 to 8 feet in diameter and 120 to 180 feet in length, rotating at a speed of from 20 to 50 seconds per revolution, a feed rate of l to 3 tons per depth of charge for effective tion. Where calcination is defluorination.

Calcination may be carried out in two stages. The first stage, for convenience called the calcining burn, may be carried out in a relatively short rotary kiln, e. g. 60 feet, at a temperature of 2000-2500 F. with a rate of feed up to 20 tons per hour. The second stage, for convenience called the defluorinating burn, may then be carried out in amuch longer kiln, e. g. 140 feet, at a temperature of 2600-2800 F. with a feed rate up to 5 tons per hour.

The following example illustrates apractice of the invention on a commercial scale in a rotary kiln 160 feet long and 6 feet in diameter, with a pitch of inch per foot and a kiln speed of about 1 revolution in seconds, a feed rate of about 1.8 tons .per hour, and a hot zone temperature of approximately 2730 F. Calcination was conducted in one stage, that is in'a single pass through the kiln. The phosphate rock was an unground plant concentrate and was'fed wet (about 11% H O) to the feed (reaction product of sodium carbonate and aqueous solution of phosphoric acid) was fed to the chute at the cold end of the kiln. The.

- the reaction product).

rotary kiln. The first pass was through with the rock and sufiicient additional mixed rock and reagent Chemical analysis P Lnsol. GaO F Percent Percent Percent Percent 34. 55 3. 5 49. 77 3. 96

Screen analysis +35 mesh --'35+1(J0 -100 Percent Percent Percent 29. 3 64. 7 7

The weight ratio of Na O to P 0 in the reaction product was approximately 1, and the calcining charge was the mixture of rock and reaction product in'the proportions by weight of about 92.5% rock (on a dry basis) and about 7.5% of added Na O and P 0 (derived from The calcining charge (feed) had the following average analysis and molar formula:

P10; Insol. OaO N820 Per- Per- Pcrcent cent Percent cent 35. 75 3.15 44. 77 3. 64

M015 0370 NBI O 3PgO5 1 Mols SiO The calcine or clinker had the following average analysis:

P205 Bio: 080 N820 F Per- Per- Per- Percent cent Percent cent cent 39. 31 3. 40 49. 22 3. 90 0.15

The P 0 availables of the calcine or clinker were as The following example illustrates a practice of the invention on a commercial scale in which calcination was carried out in two stages, that is in two passes through a a rotary kiln feet long and 8 feet in diameter. This kiln was much longer than required, or desirable, but was the only one available at the time of the run. The second pass was through the rotary kiln feet long and 6 feet in diameter) hereinbefore described in connection with the preceding example. The calcining charge, on a weight basis, was made up as follows:

Percent Phosphate rock (unground plant concentrate) 92.42

'Na O (added in reaction product) P 0 (added in reaction product) asses? in connection with the preceding .example, the slurry containing about 30% of water. The average calcining temperature of the first pass was 2300 F. The average rate of feed was 11.5 tons of rock per hour, and the average rate of clinker discharge was 10.3 tons per hour.

The recovery of phosphate in the clinker averaged 89.6%. The chemical and screen analyses were as follows, the molar formula ratio of the calculated feed being 1.68. l

The cakes and lumps of the first pass clinker were crushed to mostly minus A inch (3 mesh), and the crushed clinker was fed dry, in the second pass, at an average rate of 2.80 tons per hour. The clinker or final calcine discharge was at the average rate of 2.34 tons per hour, and the recovery of phosphate in the clinker averaged 83.6%. The average calcining temperature of the second pass was 273l F. The screen analysis of the crushed clinker (feed), the chemical analyses of the feed and final calcine, and the P availables of the final calcine were as follows:

Screen analysis of feed P 0 availables of final calcine 2% citric Neutral 0.4% H01 acid ammonium citrate Percent Percent Percent 38. 39 34. 36 34. 01

The principal advantage of carrying out the calcination in two passes is that the soft cakes and lumps, which are formed at the relatively low temperature of the first pass, are easily broken down by mechanical crusher-s before going to the second pass where the temperature employed would otherwise cause the cakes and lumps to fuse or glaze to such a degree that satisfactory defluorination cannot take place. Once these cakes and lumps are broken down they have little tendency to reform, consequently a higher feed rate can be employed in the second pass than in a single stage operation in the same kiln. The increased output results in a lower unit cost of the finished product. Since the first pass greatly reduces the tendency of the charge to form or reform cakes and lumps, the operation in the second pass, which is the dei'luorinatiug burn, is much smoother. In other Words, most of the operating difiiculties are confined to the first pass or calcining burn, where their deleterious effects are of small if any practical significance, and the critical second pass or defluorinating burn may consequently be carried out with almost complete freedom from operating difiiculties.

In practicing the invention on run-of-mine rock of fairly constant grade (i. e. P 0 content), it is usually more convenient and practical to determine the composition of the reagent mixture on the basis of the insol. (silica) content of the rock, rather than on the basis of the aforementioned molar formula. Thus, with a phosphate rock running around 35% P 0 (say 33 to 36% P 0 the composition of the reagent mixture can readily be determined from the following table on the basis of the insol. content of the rock:

Broad range Preferred range Percent insol.

Weight M01 ratio, Weight Moi ratio,

ratio, N a O/Pz 0s ratio, N mo/P205 It will be seen that as the silica (insol.) content of the rock increases from 2 to 6%, the preferred mol ratio of Na 0/P O increases from 1.90 to 2.50; an increase of 0.15 for each increase of 1% in the silica content of the rock. For the ratio to thus increase, the P 0 (in the reagent mixture) must decrease in relation to the Na O, and this decrease in P 0 will be proportional to the increase in silica content of the rock. Thus, in making up the reagent mixture from the foregoing table it is possible to avoid too greatly deviating from the charac teristic molar formula of the invention.

Where the silica (insol.) content of the run-of-mine rock (containing 33-36% P 0 is also fairly constant, and less than 4%, the following table is conveniently used in practice to determine the Na O/P O ratio of the reagent mixture, the amount of added P 0 (in percent by Weight on the combined Weight of rock and added P 0 and Na O) being determined to a large extent on the current availability of phosphoric acid, and being preferably between 5 and 6%.

Percent of N ago/P205 added ratio by P205 weight will be 4.2% and the balance of the calcining charge (90.8%) will be phosphate rock.

Throughout this specification and the appended claims, sizing is expressed in terms of Tyler standard screenscale sieves, and without substantial fusion means in the absence of such fusion or sintering as to cause the charge to become sticky, in whole or in part, and tend to cling or stick to the wall of the calcining apparatus, and, in a rotary kiln, to ball-up and to fail to flow freely and easily through the kiln. By substantially defluorinated is meant a phosphate product containing less than 1 part of fluorine per 40 parts of phosphorus. The proportioning of the phosphate rock and reagent mixture in making up the calcining charge, herein variously specified, is to be understood as applying to the charge as initially fed to the calcining apparatus.

We claim:

1. The method of defluorinating phosphate rock which comprises subjecting the rock with a silica content not exceeding 6% to calcination at a temperature of at least 2600 F. Without substantial fusion in the presence of water vapor and of a reagent mixture consisting essentially of the reaction product of sodium carbonate and phosphoric acid, the mol ratio of the Na O to P content of the reaction product being between 1.6 and 2.8, and the C210, Na O, P 0 and SiO content of the combined phosphate rock and reagent mixture being such that the mol ratio of these constituents in the formula Mols CaO+Na203P205 Mols SiO 8 is between 1.3 and 2.8, and maintaining the calcining charge at said calcining temperature for a sufficient period of time to produce a phosphate product having high fertilizer availability and containing less than one part of fluorine for each parts of phosphorus.

2. The method of claim 1 in which the molar formula ratio is between 1.6 and 2.

3. The method of claim 2 in which the mol ratio of Na O to P 0 in the reaction product is about 2.3.

4. The method of claim 1 in which calcination of the phosphate rock and reagent mixture is carried out in two stages each without substantial fusion and in the presence of water vapor, the first stage of calcination is carried out at a temperature between 2000 and 2500" F., the clinker from the first stage of calcination is crushed to mostly minus 3 mesh, and the crushed clinker is subjected to the second stage of calcination at a temperature of at least 2600 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,058,249 Meriwether Apr. 8, 1913 2,093,176 Tromel Sept. 14, 1937 2,337,498 Ritter Dec. 21, 1943 2,442,969 Butt June 8, 1948 2,556,541 Hollingsworth June 12, 1951 2,562,718 Hollingsworth July 31, 1951 FOREIGN PATENTS 453,646 Great Britain Sept. 11, 1936 

1. THE METHOD OF DEFLUORINATING PHOSPHATE ROCK WHICH COMPRISES SUBJECTING THE ROCK WITH A SILICA CONTENT NOT EXCEEDING 6% TO CALCINATION AT A TEMPERATURE OF AT LEAST 2600*F. WITHOUT SUBSTANTIAL FUSION IN THE PRESENCE OF WATER VAPOR AND A REAGENT MIXTURE CONSISTING ESSENTIALLY OF THE REACTION PRODUCT OF SODIUM CARBONATE AND PHOSPHORIC ACID, THE MOL RATIO OF THE NA2O TO P2O5 CONTENT OF THE REACTION PRODUCT BEING BETWEEN 1.6 AND 2.8, AND THE CAO, NA2O, P2O5 AND SIO2 CONTENT OF THE COMBINED PHOSPHATE ROCK AND REAGENT MIXTURE BEING SUCH THAT THE MOL RATIO OF THESE CONSTITUENTS IN THE FORMULA 